Corrosion protection of high performance steels without sacrificing their mechanical and tribological properties is critical for advanced applications such as defense vehicles and aircraft. Current materials for bearings and gears for example, have insufficient corrosion resistance to satisfy modern Navy requirements, and require corrosion inhibiting oils for corrosion resistance. Although the currently available steels are therefore able to provide corrosion protection, the oil formulations reduce boundary lubrication performance. The reduction in wear resistance, in turn, limits operating power required for improved aero propulsion engine and gear box systems. Advanced bearing steels provide adequate wear resistance, but lack corrosion resistance. High nitrogen martensitic stainless steels have improved corrosion resistance, and also exhibit good contact fatigue resistance. However, these steels are limited in abrasive and adhesive wear resistance and are subject to high speed and high temperature scuffing. The problem is that the protective chromium oxide film limits boundary lubricating film formation. Rather than chromium, iron or iron oxide is required to react with the oil additives. In addition, the high nitrogen martensitic stainless steels do not have the shear stability to resist adhesive wear or scuffing.
Nitriding involves the diffusion of nitrogen into the surface of certain steels to form compounds. In doing so, it expands the steel lattice structure, putting the atomic bonds into tension. This stress makes the surface very hard and also improves the fatigue strength. Fatigue occurs when a cycle of tension followed by compression continues for many cycles. Stressing the lattice structure causes the surface to be in compression, thereby eliminating the tension portion of the fatigue cycle.
In the gas nitriding of stainless steel, the donor is a nitrogen-rich gas such as ammonia (NH3), and is often referred to in the art as ammonia nitriding. When ammonia comes into contact with the heated work piece, it disassociates into nitrogen and hydrogen. The nitrogen concentrated on the surface then diffuses from the surface to the interior, depending upon concentration and time. This aspect of the process is well known in the art. The present invention however, comprises a process that can be accurately controlled. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular stainless steel properties required. The advantages of gas nitriding over the other variants are:                The process results in a homogeneous deposition of nitrogen on the surface.        The preparation of large batch sizes is possible—the limiting factor being furnace size and gas flow        With modern computer control of the atmosphere, the nitriding results can be tightly controlled        Relatively cheap equipment cost—especially compared with plasma nitriding        
The disadvantages of the gas nitriding processes known in the art are:                Reaction kinetics are heavily influenced by the stainless steel surface condition. An oily stainless steel surface or one contaminated with cutting fluids will adversely affect the process and result in a poor product.        Surface activation is sometimes required to successfully treat steels with a highly concentrated ammonia as the nitriding medium. Secondly, although not especially toxic, ammonia can be harmful when inhaled in large quantities. Also, care must be taken when heating in the presence of oxygen to reduce the risk of explosion        
The present invention comprises a nitrogen alloyed stainless steel with a surface treatment that affords superior performance to current martensitic stainless steels. The nitrogen alloyed stainless steels possess improved wear resistance, especially to adhesive wear and scuffing, without the sacrifice of corrosion resistance inherent in martensitic stainless steels. This is achieved through the development of a number of alloys with reduced carbon content to limit sensitization, and through the alloying of the stainless steel alloy with nitrogen. The present invention utilizes a powder metallurgy technology to improve corrosion resistance and enhance adhesion and scuffing resistance. The chromium content is increased to add to the corrosion protection, and copper is added to limit the reaction with the corrosion inhibiting oils. In addition, a nitriding or carburizing cycle was designed to further protect the surface from adhesive wear and to resist scuffing. Thus, the nitrided stainless steel alloys of the present invention have particular applications in the manufacture and assembly of Naval and Air Force aircraft tail hook catch mechanisms.